Lab 4: DSC
1. Purpose
Examine thermal transitions in a semi crystalline polymer Poly (ethylene terephthalate)
sample using DSC.
2. Theory
Differential scanning calorimeters (DSCs) are used to measure the temperatures and heat
flow during thermal transitions of materials in a wide range of applications such as polymer
characterization, food science, and pharmaceuticals.
Fig 1: Schematic representation of DSC sample cell.
The sample to be characterized is sealed in an aluminum pan and placed alongside an
empty aluminum reference pan in the DSC furnace. The sample and the reference pan are heated
at the same rate by a heating element in the furnace. Thermal transitions such as melting or
crystallization involve release or absorption of heat which leads to a difference in temperature
between the sample and reference. The DSC measures the difference in heat flow required to
maintain the sample and the reference at the same temperature. A purge gas such as helium or
nitrogen may be used to ensure thermal efficiency and to remove left over volatiles.
DSC is the most commonly used instrument for characterization of thermal transitions in
polymeric materials. Completely amorphous polymers such as polystyrene can be characterized
using their glass transition temperature (Tg) which represents a transition from a rigid glassy state
to a rubbery liquid like state. In DSC measurements the glass transition is captured by a step
change in the heat flow curve. Since amorphous polymers soften considerably beyond their Tg, it
often determines the end use temperature range for these materials. Semi crystalline polymers
such as poly (ethylene terephthalate) which is often referred as PET consist of amorphous and
crystalline domains and exhibit a glass transition and a melting transition. Furthermore,
depending on processing conditions semi crystalline materials may also exhibit a crystallization
peak on heating, as shown in Figure 2a.
a)
b)
Fig 2: Thermal transitions in a semi crystalline polymer a) crystallization on heating b) No
crystallization on heating ( T. D' Amico, C.J. Donahue, E.A. Rais, J. Chem. Ed. (2008) 85, 404).
The degree of crystallinity is an important parameter for semi crystalline polymers. The
degree of crystallinity can be determined from the crystallization exotherm when crystallization
occurs on heating (see Fig 2a) using the Equation 1 below:
Degree of crystallinity =
H f  H c
H f 100%
100%
(1)
where H f is the enthalpy of melting, H c is the enthalpy of crystallization, and H f 100% is the
enthalpy of melting for a fully crystalline polymer. When no crystallization is observed on
heating (See Fig 2b), the degree of crystallinity may be estimated from the melting peak as
shown in Equation 2:
Degree of crystallinity =
H f
H f 100%
100%
(2)
The degree of crystallinity is influenced by processing conditions and thermal history of the
polymer. For example, when a semi-crystalline polymer is cooled from the melt, the degree of
crystallinity increases with decreasing cooling rates. In this lab, the impact of two different
cooling procedures on the thermal properties of a semi crystalline polymer is examined.
3.
Procedure
3.1
Thermal Treatments
1)
Cut three round pieces from a PET beverage bottle using punch tool and measure their
masses using the weighing balance.
2)
Place one piece on a standard aluminum pan and cover with lid. To close the sample
using the sample press, place the pan on the green die and position it under the press.
Gently pull the handle forward to close the pan. Repeat the sample preparation procedure
for one more PET sample. Place the remaining PET piece in an aluminum pan and do
not cover with this lid (This sample will be used to observe the extent of melting during
thermal treatments). NOTE: Remember to keep track of sample mass in each pan!
3)
Place the uncovered sample pan and the two covered sample pans on the top on the hot
plate. Set the hot plate temperature to 300 oC. Observe the extent of melting of the PET
sample in the uncovered pan. This can be used as a reference to track the extent of
melting in the two covered pans.
4)
Transfer some liquid nitrogen from the dewar to the green holding container.
4)
When the PET sample in the uncovered pan is in a liquid-like melt, switch off the hot
plate and using tweezers quickly transfer one of the covered sample pans to the liquid
nitrogen container. Let the other covered sample pan remain on the hot plate to cool
slowly.
5)
After the sample pan is allowed to remain in the liquid nitrogen contained for a few
minutes, proceed to perform DSC measurement. Finally, perform a DSC measurement
for the covered sample pan that was allowed to cool slowly on the hot top.
3.2
DSC measurements
Sample Preparation
Sample loading: use tweezers to place sample pan onto DSC cell closest to table edge (see
photo) and reference pan (empty, covered aluminum pan) on DSC cell farthest from table
edge. Replace both inner lids using the curved backside of the tweezers, and replace
outmost lid by hand.
Pre-test System Checks:
1.
Purging gas cylinder (Nitrogen, shared with TGA Q50): gas level and open valve to DSC
2.
Open TA Instrument Explorer and double-click Q20 icon. Check Purge gas flow rate
(50 ml/min)
Software Inputs:
1. Summary tab
Mode: Standard
Test: Custom
Sample Name: <give descriptive sample name>
Pan Type: Aluminum
Sample Size: <sample mass>
Comments: <give additional sample information> i.e. slow cooled, quenched etc.
Data File Name: \\GI3113-01\TA\Data\DSC\Users\yourfolder\your_file_name
NOTE: file name should first include initials, then descriptive information. (Ex:
John Andrew Smith’s 1:1 DCPD:dilulin sample: jasDilulinDCPD50% ).
2.
Procedure tab
Test: Custom
Editor: Segment description: Ramp 20 K/min to 275 oC. (The same condition must be
maintained for every experiment)
3
Notes tab
Operator: <your name>
Extended Text: <any additional information you would like to record>
Sample: #1 Nitrogen
Flow Rate: 50 mL/min
4.
Click Apply to save changes to save changes.
5.
When sample is loaded, start the experiment by pressing Start the Run button in the upper
left corner of the screen.
Post-test:
1.
To cool the DSC down after a test, replace the DSC cell cover with the quench cool
accessory. Pour liquid nitrogen from the dewar carefully in the quench cool accessory until
the desired temperature is attained.
2.
When run is complete and DSC is cooled, close the nitrogen gas valve and dispose of the
sample.
3.
The experimental results such as glass transition temperature, melting temperature, cold
crystallization temperature, and enthalpy of melting may be obtained by opening the data
file in universal analysis software. After opening the data file use the analyze tab to obtain
the necessary values.
4. Assignment
1. Do both specimens exhibit a cold crystallization peak? Explain the reasons for differences, if
any.
2.
Calculate the degree of crystallinity for the samples tested using the appropriate equations.
Are the degrees of crystallinity for the two specimens different? If so explain the origins of
the difference? The value of H f 100% is 140.1 J/g.
3. Compare the glass transition temperatures of the two specimens. Is there a relationship
between the glass transition temperature and degree of crystallinity?
4. Determine the melting point of each specimen? Does melting point depend on degree of
crystallinity?
5. Does the observed enthalpy of melting depend on the degree of crystallinity of the specimen?